1. Field of the Invention
The present invention relates generally to radiant flux sensors and, more specifically, to radiant flux sensing methods and apparatus which utilize radiant flux feedback to reduce response excursion.
2. Brief Description of Related Art
Many electromagnetic radiation sensors respond to input changes more slowly than desired. Energy storage within the sensor causes this delay. Physical mechanisms which store energy include electrical capacitance, semiconductor junction charge, and thermal capacitance. A detected signal must supply energy to the storage mechanism before the sensor's output can reach a full response. Conversely, when the input signal is removed, energy must be drained from storage before the sensor's output can subside.
A transimpedance amplifier (TIA), shown in FIG. 1, offered the best solution prior to this invention. A radiant flux sensor 24 receives an external signal radiant flux Hs. Radiant flux has units of watts. Radiant flux is the flow of electromagnetic energy onto (or through) a surface.
Sensor 24 produces a small current Ii in response to signal Hs. Current Ii creates a small voltage Vi at circuit node 20. The open-loop gain of amplifier 26 increases voltage Vi, producing an output voltage Vo. A feedback resistor Rf controls the closed-loop gain. Resistor Rf commonly has a large resistance, in the range of 1 megohms (1e6) to 1 teraohms (1e12).
Voltage Vo, a negative voltage, causes a current Ii' to flow in resistor Rf. Current Ii' is substantially equal to current Ii, differing only by the relatively small input current of amplifier 26.
In operation, output Vo moves through a relatively large voltage swing while input Vi moves a barely measurable swing. The high gain of amplifier 26 makes voltage Vi vanishingly small. Sensor 24 behaves as if its output current Ii were shunted by a resistance which is low compared to resistance Rf. The apparent shunt resistance is resistance Rf divided by the open-loop gain of amplifier 26.
A TIA greatly decreases the sensor output voltage excursion (change) which corresponds to a given signal change. A smaller output change means less energy must be stored or removed during the response. The TIA output responds in a shorter time because the sensor signal can satisfy the reduced energy storage change more quickly.